1. Field of the Invention
This invention relates to an improved process for forming a layer of aluminum on a semiconductor wafer. More particularly, this invention relates to a multistep sputtering process for depositing a layer of aluminum over a surface on a semiconductor wafer having closely spaced apart raised portions.
2. Description of the Related Art
In the formation of an aluminum layer by sputtering to deposit aluminum over a stepped surface, e.g., where the integrated circuit structure includes spaced lines, trenches, and/or vias or contact holes on a semiconductor wafer, where the spacing is small, e.g., the spaced lines or trench walls are less than 1.6 micrometers (microns) apart or the via diameters are less than 1.6 micrometers, aluminum layers formed by conventional sputtering processes not only are not planar, but such aluminum layers actually are thinner in the regions between such closely spaced apart raised portions, i.e., between steps or trench walls or over the vias.
This is thought to be due to a shadowing effect wherein the raised portions or steps on either side of the narrow regions prevent or inhibit deposition of the sputtered atoms from reaching such regions unless the atom reaches the wafer from a path which is nearly perpendicular to the wafer. That is, sputtered atoms traveling at an angle toward the wafer encounter the raised steps, or sidewalls of the trench or via, and deposit thereon without reaching the low regions on the wafer between such closely or narrowly spaced apart raised portions.
FIG. 1, which is illustrative of the results obtained using such prior art processes, shows the effects of shadowing when steps on a semiconductor wafer are closely spaced apart. A semiconductor wafer 10 is shown having steps 11, 12, and 13 formed on the wafer prior to the sputtering of an aluminum layer 20 thereon. Aluminum layer 20 does not fill narrow region 14 between closely spaced apart steps 11 and 12, but rather only forms a thin layer portion 22 which is thinner than the remainder of aluminum layer 20, because region 14 is shadowed by the adjacent high steps 11 and 12. In contrast, the wider region 16 between steps 12 and 13, which are not closely spaced apart, is completely filled with aluminum which, as seen at 24, is not thinner than the remainder of layer 20, presumably because there is no shadowing effect here as there is in region 14.
In addition to the difference in thickness between the aluminum deposited in regions 14 and 16, it will be noted that aluminum layer 20 forms a thin layer at 26 on the sidewalls of narrow region 14 and that aluminum layer 20 then gradually slopes inwardly, in what made be termed a "negative" slope, to define an overhang or inward necking of aluminum layer 20 at 28 in prior art FIG. 1. While this effect will also be noted slightly in aluminum layer 20 at 24 over region 16 in FIG. 1, the effect over such wide regions is negligible. However, this negative slope effect in regions between closely spaced apart raised portions of a semiconductor wafer, such as shown over region 14 in FIG. 1, can result in the formation of voids in the aluminum layer as well as add to the shadowing effect by further hindering the passage of sputtered aluminum into region 14.
Since integrated circuit structures formed on semiconductor wafers are becoming smaller and smaller with finer features, including thinner lines, smaller line pitches, and smaller contact holes, it is important to be able to overcome this shadowing effect and negative slope deposition of aluminum in low areas between closely spaced apart raised portions of a semiconductor wafer; and to be able to sputter an aluminum layer on a wafer surface containing closely spaced apart raised portions, which will not form such thin films or layers in the regions between the closely spaced apart raised portions of the integrated circuit structure, but will rather substantially completely fill in such low areas between closely spaced apart raised portions of a semiconductor wafer and which will not form negative slopes during the aluminum deposition.